This invention relates to heat sealable blends of polypropylene terpolymers containing ethylene and 1-butene with copolymers of ethylene and alpha olefins containing four or more carbon atoms. Such blends may be used to form film which is characterized by lower heat seal initiation temperatures than films known in the past.
A wide variety of plastic films are used in packaging applications such as bags, pouches, tubs and trays. In many of these applications, it is important that the plastic film be readily heat sealable as well as possessing other good physical and mechanical properties such as resistance to tearing, tensile strength, and processability in high speed equipment.
Many plastic materials which are used in the formation of film products do not themselves exhibit good heat sealing characteristics. Polypropylene, for instance, is not readily heat sealable at the relatively low temperatures which are preferred for use in commercial film making applications. As a result, there has been quite a bit of development work to find plastic materials which are readily heat sealable and which can be used compatibly with stronger substrate materials to form multilayer films.
It has been discovered that polypropylene polymers such as terpolymers of primarily propylene units with smaller amounts of ethylene units and units of an alpha olefin having four or more carbon atoms exhibit excellent heat sealing characteristics. U.S. Pat. Nos. 4,256,774, 4,275,119, 4,303,708, 4,339,497 and 4,339,498 disclose the use of such terpolymers and other terpolymers in the manufacture of heat sealable packaging films. Blends of different polymers have been used to create good heat sealing films. U.S. Pat. Nos. 4,291,092, 4,297,411, 4,339,496 and 4,340,641 all disclose heat sealable films made from a blend of a copolymer of ethylene and a higher olefin and a copolymer of propylene and an olefin such as ethylene or one that has four or more carbon atoms. Also, U.S. Pat. No. 4,316,970 discloses a film made from a heat sealable blend of a butene-1-ethylene copolymer and about 2-9% by weight of polypropylene. This blend is said to form film exhibiting improved heat sealing characteristics.
One of the most important characteristics for a heat sealable film is the temperature at which the sealing begins, i.e. the heat seal initiation temperature. This is important because it is desired to operate at as low a temperature as possible because (1) it broadens the heat-sealable range, and (2) it permits higher productivity due to less time for cooling. I define the heat seal initiation temperature as that temperature when the strength of the heat seal exceeds 0.5 pounds per inch because this criterion satisfies most of the applications.
The terpolymers and blends described above all exhibit very good heat sealing characteristics. However, I have found that my blend achieves a lower heat seal initiation temperature than the terpolymers alone.